A technique of heating a sheet having an ink adhering thereto and thereby decolorizing the ink adhering to the sheet is proposed. For decolorizing the ink, a dedicated device is used to heat the sheet. Specifically, plural sheets are stacked in the dedicated device and the plural sheets are heated in the state of being nipped by the device.
If plural sheets are superimposed and then heated, heat cannot be efficiently transmitted to the ink on each sheet. That is, if heat is to be transmitted to plural sheets which are superimposed on each other, heat is transmitted most quickly to the sheet situated at the edge in the sheet stacking direction and heat is transmitted most slowly to the sheet situated in the middle.
Such heat transmission may cause more than necessary heat to be transmitted to a specific sheet and heat energy is wastefully consumed.